1. Field of the Invention
This invention relates generally to propulsion systems and more particularly to a dual mode hybrid electric thruster.
2. Description of the Related Art
Spacecraft such as communications satellites typically utilize electric thrusters for stationkeeping and other functions. Electric thrusters, such as ion thrusters utilize electrical power generated by the solar cells of the satellite to supply energy to a propellant to achieve the propulsion. The ion thruster also has a high specific impulse, making it an efficient engine which requires very little propellant. As a result, ion thrusters require relatively small amounts of a consumable propellant, which is an advantage over using an engine using chemical propellants.
Ion propulsion generally involves employing an ionized gas accelerated electrically across charged grids to develop thrust. The electrically accelerated particles can achieve speeds of approximately 30 km/second. The gas used is typically a noble gas, such as xenon. The principal advantage afforded by ion propulsion systems over conventional chemical propulsion systems is their very high efficiency. For example, with the same amount of fuel mass an ion propulsion system can achieve a final velocity as much as ten times higher than that obtainable with a chemical propulsion system.
FIG. 1 is a schematic diagram of a prior art ion thruster 100. In operation, ionized gas 102 is accelerated across an ion-optics system 104. The ionized gas 102 is a plasma of ions and electrons that is created and confined within the body of the ion thruster. Ions from the plasma are electrostatically accelerated rearwardly by ion-optics system 104, driving the spacecraft forward in the opposite direction. The ion-optics system 104 includes grids to which appropriate voltages are applied in order to accelerate the ions rearwardly. The grids are in a facing orientation to each other, spaced apart by relatively small clearances such as about 0.035 inches at room temperature. The grids include aligned apertures therethrough. The ions accelerated by the applied voltages pass through the apertures, providing the propulsion.
Ion propulsion is well suited for space applications where low thrust is often acceptable and fuel efficiency is critical. More and more ion propulsion is becoming a component of new spacecraft designs. Spacecraft, including satellites as well as exploration vehicles, are presently making use of ion propulsion systems. For example, ion thrusters are currently used for spacecraft control on some communications satellites. Some existing systems operate by ionizing xenon gas and accelerating it across two or three charged molybdenum grids.
Unfortunately, the range of ion propulsion applications is narrowed by the fact that, although they are efficient, ion propulsion systems develop very low thrust when compared with chemical propulsion systems. Chemical propulsion systems create thrust by thermodynamically expanding heated propellant gas through a nozzle. The energy to heat the propellant is stored in the chemical bonds of the propellant or propellant/oxidiser combination and released through decomposition in single propellant systems or chemical reaction in multi-propellant systems.
Chemical propulsion systems are generally, solid fuel propulsion systems, fluid fuel propulsion systems, or cold gas propulsion systems. Solid fuel propulsion systems provide large amounts of thrust, but cannot be shut off once ignited. The fuel and the catalyst combine to generate high temperatures and pressures. The gaseous fuel then passes through a nozzle and is expelled. The release of the hot gas creates an equal and opposite force pushing the engine forward. Higher combustion temperatures result in greater thrust. A solid fuel engine burns from the inside out, and as a result, the cavity inside the engine is getting larger.
Liquid fuel propulsion systems typically provide less thrust per kilogram of fuel than solid fuel propulsion systems and are much more volatile. Two liquids are combined inside of a combustion chamber and ignited. The resulting gas is expelled through a nozzle. Unlike solid fueled engines, liquid fueled engines can be turned off and on whenever they are needed and the efficiency reduction over the lifetime of the engine is smaller.
Cold gas propulsion systems generally include a single gas and a nozzle. Whenever thrust is needed the nozzle is opened and some of the gas is expelled. Cold gas systems do not produce as much thrust per kilogram of fuel as liquid or solid fueled systems due to the lack of combustion. The thrust and the efficiency rely solely on the pressure in the containment tank. As the cold gas system is used the pressure goes down and so does thrust and efficiency.
Chemical propulsion systems are limited by the available reaction energies and thermal transfer considerations to exhaust gas velocities of a few thousand metres per second. As a result, spacecraft typically use chemical thrusters to provide high thrust at a low Isp for orbit raising and altitude and orbit control subsystems (AOCS). When low thrust is ideal, such as during station/attitude control, spacecraft, when so equipped, employ electric thrusters at a high Isp.
Typical missions therefore often utilize spacecraft carrying both thruster types at a cost and mass disadvantage. To avoid the mass disadvantages of carrying low-efficiency fuel for (say) orbit raising from initial transfer/parking orbit to final altitude, one other prior art solution has been to suggest the application of low thrust ion engines to this orbit raising phase, but with compromise in extended periods of transportation (up to several months), with risk of Van Allen belt radiation damage amongst other concerns. Another prior art design has been a two-stage ion thruster which applies two different voltages for ion extraction. This design allows for some level of thrust control, but the resultant increase thrust is very small.
In view of the foregoing, there is a need for a dual mode propulsion system. The system should be throttleable. Thus, the system should provide higher thrust at times needing more rapid motion, such as during orbit raising, and reverting to lower thrust and higher fuel efficiency usage at times when low thrust is ideal, such as during station/attitude control.